Method and apparatus for regenerating symbol timing from a probing signal in a system having non-linear network and codec distortion

Abstract

The present invention provides a method of generating a signal that may be used to determine the characteristic response of a communication channel that utilizes the public Digital Telephone Network (DTN). The channel includes the DTN, which may have Network Digital Attenuators (NDA) and/or Robbed Bit Signalling (RBS), and a Digital-to-Analog Converter (DAC), (also known as a codec), as well as the analog characteristics of the local loop, typically a twisted pair of copper wires. The present invention provides a method and apparatus to determine the optimal sampling instant of the received data stream. The present invention provides a probing signal that is well-suited for use in determining the channel's response to a known sequence of PCM codes used as data symbols. This is especially useful in so-called PCM modulation schemes that utilize the DTN, where knowledge of network and DAC distortion predicates the selection of available PCM codes used to represent data. This information is also useful when the data receiver, or PCM modem, makes determinations of which codes were actually sent.

We claim: 
     
       1. A method of recovering symbol timing information from a probing signal sent over a communications channel between a digital data source and an analog subscriber connected to a digital telephone network by an analog loop comprising the steps of: receiving a probing signal having positive and negative pulses separated by intervening periods of zero values;   filtering said probing signal by two frequency selective and phase selective filters adapted to extract timing and phase information;   squaring the output of the filters;   determining symbol timing phase error information from the squared outputs of the filters; and,   controlling a voltage controlled oscillator in response to the symbol timing phase information.   
     
     
       2. A method as in claim 1 where the filtering step is performed using two FIR filters that selectively pass frequencies at one half the symbol rate. 
     
     
       3. A method as in claim 2 wherein one FIR filter has filter coefficients corresponding to a sampled cosine wave, and the other FIR filter has filter coefficients of a cosine wave offset by 45 degrees. 
     
     
       4. A method as in claim 3 wherein the coefficients of one FIR filter are given by cos(kπ/2+φ), and the coefficients of the other FIR filter are given by cos(kπ/2-π/4+φ), where k=0,. . .,31, and 2π≧φ>0. 
     
     
       5. A method as in claim 2 wherein the filtering step further comprises the step of decimating the FIR filtered output sequences. 
     
     
       6. A method as in claim 2 wherein the determining step comprises the steps of: integrating the squared outputs over a period of time to obtain two sums, one corresponding to each squared output of the FIR filters; and   determining an arctangent of the ratio of the two sums.   
     
     
       7. A method as in claim 2 wherein the two FIR filters differ primarily from each other in their phase response. 
     
     
       8. A method as in claim 1 further comprising a step of eliminating a DC component by substracting a previous squared filter output from a current squared filter output. 
     
     
       9. A symbol timing recovery apparatus for use with a probing signal sent over a communication channel between a digital data source and an analog subscriber connected to a digital telephone network by an analog loop comprising: an analog-to-digital converter adapted for connection to the analog loop;   first and second frequency selective and phase selective FIR filters adapted to extract timing and phase information connected to said analog-to-digital converter;   first and second squaring circuits connected to said first and second frequency selective and phase selective FIR filters;   first and second DC eliminator circuits connected to said first and second squaring circuits;   first and second low-pass filters connected to said first and second DC eliminator circuits;   error signal generator connected to said first and second low-pass filters for generating a timing error signal; and,   a voltage controlled oscillator connected to said error signal generator for altering the sampling instant of said analog-to-digital converter in response to said timing error signal.   
     
     
       10. A symbol timing recovery apparatus of claim 9 wherein said first and second FIR filters selectively pass frequencies at one half the symbol rate. 
     
     
       11. A symbol timing recovery apparatus of claim 10 wherein said first and second FIR filters have filter coefficients corresponding to samples of a cosine wave. 
     
     
       12. A symbol timing recovery apparatus of claim 11 wherein said first FIR filter has filter coefficients derived by the equation cos(kπ/2+φ), and the coefficients of said second FIR filter are given by cos(kπ/2-π/4+φ), where k=0,...,31, and 2π≧φ>0.